Numerous different types of in-package sulphur dioxide generating devices have been proposed and various of these are in commercial use. Whilst existing sulphur dioxide generating devices are effective to some extent there are deficiencies in at least some respects.
The first general difficulty is to obtain an effective balance between the amount of sulphur dioxide produced; the rate at which it is released; and the length of time during which satisfactory release of sulphur dioxide can be achieved. Many commercially available sulphur dioxide generating devices give off a satisfactory amount of sulphur dioxide gas in the early stages but the SO2 production decreases to an unsatisfactorily low level too soon.
A solution to this problem can be achieved by mixing the chemically active compound with a plastisol or other matrix forming material and curing it with the active compound in particulate or powder form substantially evenly distributed throughout its mass. Generally the plastisol mixture is applied as a layer to a sheet forming a substrate and is covered by another sheet to form a three-layered laminate. Both sheets are pervious to moisture and sulphur dioxide gas. By ensuring adequate but controlled access by moisture in the surroundings to the chemical compound within the plastisol mass (i.e. in the plastic bag in actual use) the quantity of sulphur dioxide produced, and the rate at which it is produced, can be controlled to an advantageous extent.
Control is achieved by selecting the various variables including the thickness of the layer of mixture; the amount of chemical compound mixed with the plastisol; and the means employed for enabling moisture to penetrate the cured plastisol mass at a rate commensurate with the required rate of production of sulphur dioxide over a required period of time. Clearly there also needs to be the facility for enabling sulphur dioxide generated within the cured plastisol mass to migrate from the mass to the atmosphere in the packaging bag in order that it may carry out its preservative action. Such facility will generally be inherently present but it may also require separate consideration.
The means for enabling moisture to penetrate the plastisol mass can be the provision of permeable particles in the mass which substantially touch each other and thus form a path for moisture to enter the mass. Alternatively, or in addition, small gas bubbles can be either generated within the plastisol during the curing process by using a blowing agent or the like, or air may be mixed into the plastisol during the blending of the plastisol and other ingredients together. In the latter case the gas is typically air. A humectant may also be mixed into the plastisol mass in order to attract moisture to the interior thereof. The resultant laminate is then cut up into pieces of a size suitable for inclusion in a package. A composition of this general type forms the basis of South African patent number 96/2517.
However, production of the laminate is difficult to control and, in addition, the cut pieces of laminate may have particles of chemical compound at the cut surface. The latter may have a deleterious effect on fruit which comes into direct contact with such a cut edge. Exposed chemical compound at the cut edges may also be objectionable from a health point of view.
For this reason, the laminate is sometimes placed in a moisture-permeable sleeve which is then included in the fruit package, but this increases the production costs and it is also time consuming to individually insert each laminate into a sleeve.
Accordingly, a need exists to provide an in-package preservative gas generating device which does not exhibit the disadvantages outlined above, at least to the same extent and which can be manufactured in an expeditious manner.